Organic/Inorganic Langmuir-Blodgeti Films Based on Metal Phosphonates
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of which have been demonstrated as active components in a variety of electrical, optical and magnetic materials applications [1]. Molecular LB films are most often fabricated from amphiphilic derivatives of the molecules of interest, and the traditional approach is to prepare a functionalized fatty acid. Amphiphilic carboxylic acids commonly form stable monolayers at the air/water interface and are made more processible by the presence of metal ions in the aqueous subphase. LB films prepared as salts tend to be more stable than films of neutral molecules because the ionic metal-headgroup interactions help bind the amphiphiles together. But even layered films of the metal carboxylate salts are often metastable structures, and despite the elegance of the supramolecular assembly, the long term stability of most LB films is not sufficient for materials applications. It is possible, however, to consider other metal-ligand interactions when constructing LB films, and we have recently been exploring the formation of LB films based on known organic/inorganic layered solids [2-6] such as the layered metal phosphonates shown in Figure 1. In the solid-state, organophosphonates with divalent, trivalent and tetravalent metal ions form layered structures where metal/phosphonate extended-solid layers are separated by layers of the organic group [7-91. Figure 1 shows crystal structures of two examples of solid-state metal phosphonates, Mn(03PC 6H5)'H20 and La(0 3PC6H5)(HO3PC 6H5) [8, 101. Several 461
Mat. Res. Soc. Symp. Proc. Vol. 488 ©1998 Materials Research Society
divalent metals form layered phosphonates that are isostructural with Mn(O3PC 6H5 )'H20, and similarly, La(03PC6H5)(HO 3PC6H5 ) is a prototype for a larger family of trivalent metal phosphonates. Also, layered structures are not restricted to phenylphosphonates. As the organic group is changed from a phenylphosphonate to any of a number of alkylphosphonates, the interlayer distances vary depending on the length of the alkyl chain, but the in-plane lattice constants remain virtually unchanged [8]. Since the solid-state lattice energy of the ionic/covalent metal phosphonate sheets favors a layered structure, LB films based on metal phosphonates should no longer be metastable.
Figure 1. Top. A single layer of the solid-state manganese phenylphosphoponate, Mn(0 3 PC6H 5).H 20, viewed parallel to the manganese ion plane. Crystallographic data were taken from reference [8]. Bottom. A single layer of the solid-state lanthanum phenylphosphonate, also viewed parallel to the metal ion plane. Crystallographic data were taken from reference [101.
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Another advantage of modeling LB films after organic/inorganic layered solids is the possibility of building into the films physical properties that are typical of the inorganic solid-state. We recently demonstrated this idea by characterizing a magnetic LB film [6]. LB films of manganese octadecylphosphonate undergo a magnetic ordering transition at 13.5 K to a canted antiferromagnetic state. The same magnetic ordering is se
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